The approval was the last regulatory hurdle in the purchase, first announced in March as an affiliation. Baystate, the region's largest health-care provider, has already promised to keep inpatient services in Westfield and to bring more medical specialties there from its Springfield hub.

Baystate spokesman Ben Craft said Wednesday that Baystate will share more details in the next several weeks. Noble executives have promised more details starting next week.

Changes in health care, including an emphasis on population health and preventative care versus fee-for-service and low Medicaid reimbursements make it tough on hospitals.

Noble is one of the larger employers in Westfield with 750 workers, about 100 of them are unionized members of the Massachusetts Nurses Association. The contract, which expires in March 2016, has a successor clause and will still be in force after ownership changes hands.

The move leaves Holyoke Medical Center as the only remaining unaffiliated hospital in the Pioneer Valley.

More than 25,000 patients visit Noble's emergency department each year, and they average over 4,000 inpatient discharges and 44,000 outpatient visits each year.

Noble was founded in 1883 with money from the estate of Reuben Noble, a Westfield native who'd made a fortune in the city's famous whip industry. The original Noble Hospital had 20 beds and was staffed by eight physicians. It admitted 114 patients in 1887, its first year of operation. Patients paid $7 a week for a ward room and $15 a week for a private room.